The present invention relates to an apparatus for synchronized playback of audio-video (hereafter abbreviated as AV) signals, wherein compressed digital data of AV signals are decompressed into a video signal component and an audio signal component to be represented in synchronization with each other.
In the synchronized playback of AV signals, compressed digital data of AV signals read out from recording media such as a CD-ROM (Compact Disk Read Only Memory) or a hard disk device are decompressed before being represented through output devices such as a CRT monitor and a speaker. When the apparatus for the synchronized playback is provided with data processing ability sufficiently high for the data decompression of the AV signals, or when video signal size of the compressed digital data is comparatively small, the data decompression of both of the video signal component and the audio signal component can be performed within a short time, and so, audio data and video frames can be reproduced without interruption.
However, when the data processing ability of the apparatus is not sufficiently high compared to the video signal size of the compressed digital data, the apparatus can not reproduce both the audio signal component and the video signal component in real time with which they are recorded originally, taking too much time for the data decompression processes. In consequence, the video signal component may be represented as a slow-motion film and the audio signal component is reproduced intermittently.
The intermittent reproduction of the audio signal component gives a sense of intense incongruity to the human hearing. Therefore, the audio signal component is usually processed to be reproduced continuously with a priority over the video signal component. The data decompression and displaying process of the video signal component is performed when the audio signal component is not processed. Here, adequate thin down of the picture frames (so-called frame omission) becomes necessary for synchronizing the reproduction timings of the audio signal component and the video signal component so that the video signal component may not be delayed from the audio signal component.
As for the digital data compression, there is known an MPEG (Motion Picture Experts Group) standard, wherein the audio signal component and the video signal component of the AV signals are multiplexed and compressed into digital data each added with timing information indicating reproduction time of each thereof. By referring to the timing information, the audio signal component and the video signal component are reproduced in synchronization in the decompression procedure.
As a prior art technique of the synchronized playback of the audio (voice) signal component and the video (picture) signal component, there is, for example, a Japanese patent application entitled "An apparatus for compression and reproduction of AV signals" and laid open as a Provisional Publication No. 107514/'95 (hereafter called the first prior art apparatus).
In the first prior art apparatus, color difference information is thinned down in a video data compression procedure considering the processing speed of the reproduction apparatus. Compensating the thinned down color difference information by simple interpolation, the decompression procedure of the video data is performed at a high speed, enabling synchronized reproduction of the video data and the audio data.
In another Japanese patent application entitled "Variable speed synchronized playback of recorded digital audio and video data" and laid open as a Provisional Publication No. 303240/'95 (hereafter called the second prior art apparatus), there is proposed another method of synchronized playback with a variable speed of the audio signal component and the video signal component.
In this method, a time required for decompression and playback of a frame of the video signal component, and a time required for decompression and reproduction of a block of the audio signal component are calculated from respective attribute information of each thereof. One of the calculated times is used as a master time clock for decompession of the audio signal component and the video signal component. The audio signal component and the video signal component are decompressed more speedily or more slowly according to a scaling factor of the master time clock indicated by a user.
A problem of the first prior art apparatus is that feedback control means for the synchronization are not provided. In the first prior art apparatus, the thinning down of the color difference information is performed a priori in the data compression procedure according to the information amount of the video signal component to be processed, and no means are provided for detecting lead or lag of a displayed video frame to the corresponding audio data. Therefore, unnecessary thinning down of color difference information is performed even when the playback apparatus is provided with sufficient data processing ability, and restoration of the lead or lag of the video signal component to the audio signal component can not be performed flexibly, in the first prior art apparatus.
A problem of the second prior art apparatus is that a TDHS (Time Domain Harmonic Scaling) system is used for synchronizing reproduction of the audio signal component to the scaled master time clock. The TDHS system needs a little complicated processing inconvenient to be applied to the playback system with low data processing ability, without saying of the distortion of the audio signal component.
Furthermore, when the playback apparatus is implemented with software operating on an operating system with a low resolution software timer, there is a problem that correct synchronization between the audio signal component and the video signal component cannot be obtained because of the low resolution of the software timer, even if the feedback control means are provided. That is, when the resolution of the software timer is 10 ms and decompression process of the audio signal component corresponding to one frame of the video signal component having 30 frames per second takes 13 ms, for example, decompression process of the audio signal component is measured to be accomplished in 10 ms, and 23 ms is allotted for decompression process of the video signal component of one frame. Therefore, a delay of 3 ms par frame is derived in the reproduction of the video signal component.
This is another problem of the prior art apparatus.